As will be appreciated herein below, except as otherwise indicated, aluminium alloy designations and temper designations refer to the Aluminium Association designations in Aluminium Standards and Data and the Teal Sheets Registration Record Series as published by the Aluminium Association in 2014 and frequently updated, and well known to the persons skilled in the art. For any description of alloy compositions or preferred alloy compositions, all references to percentages are by weight percent unless otherwise indicated.
7000-series aluminium alloys are aluminium alloys containing zinc as the predominate alloying ingredient other than aluminium. For purposes of the present application, 7000-series aluminium alloys are aluminium alloys having at least 2.0% Zn, and up to 10% Zn, with the zinc being the predominate alloying element other than aluminium.
In the production of motor vehicles in particular aluminium alloys the AA5000- and AA6000-series alloys like 5051, 5182, 5454, 5754, 6009, 6016, 6022, and 6111, and various others, have been used to produce automotive structural parts and body-in-white (“BIW”) parts.
There is a demand for the use of aluminium alloys which are formable and having in particular increased strength after being subjected to a paint-bake cycle. In addition, the properties normally required for such parts include a high formability for the forming operation (typically by means of stamping, deep drawing, or roll forming), high mechanical strength after paint baking so as to enabling down gauging thus minimising the weight of the part, good behaviour in the various assembly methods used in motor vehicle manufacturing such as spot welding, laser welding, laser brazing, clinching or riveting, and an acceptable cost for mass production.
There is an increasing interest for the use of 7000-series aluminium alloy products in automotive applications or other applications (e.g. railway vehicles and boats) taking benefit of the relative light weight of the aluminium alloy in combination with its high strength. For example international patent application WO-2010/049445-A1 (Aleris) discloses a structural automotive component made from an aluminium alloy sheet product having a gauge in a range of 0.5 to 4 mm, and having a composition consisting of, in wt. %: Zn 5.0-7.0%, Mg 1.5-2.3%, Cu max. 0.20%, Zr 0.05-0.25%, optionally Mn and/or Cr, Ti max. 0.15%, Fe max. 0.4%, Si max. 0.3%, and balance is made by impurities and aluminium. The sheet product has been solution heat treated (“SHT”) and cooled, artificially aged, after aging formed in a shaping operation to obtain a structural automotive component of predetermined shape, and subsequently assembled with one or more other metal parts to form an assembly forming a motor vehicle component, and subjected a paint-bake cycle.
A conventional process for producing 7000-series aluminium alloy products in rolled form includes the processing steps wherein a 7000-series aluminium alloy body is cast, after which it is homogenized and then hot rolled to an intermediate gauge. Next, the 7000-series aluminium alloy body is cold rolled after which it is solution heat treated and quenched, for example by means of water such as water quenching or water spray quenching. “Solution heat treating and quenching” and the like, generally referred to herein as “solutionizing”, means heating an aluminium alloy body to a suitable temperature, generally above the solvus temperature, holding at that temperature long enough to allow soluble elements to enter into solid solution, and cooling rapidly enough to hold the elements in solid solution. The suitable temperature is alloy dependent and is commonly in a range of about 430° C. to 560° C. The solid solution formed at high temperature may be retained in a supersaturated state by cooling with sufficient rapidity to restrict the precipitation of the solute atoms as coarse, incoherent particles. After solutionizing, the 7000-series aluminium alloy body may be optionally stretched a small amount (e.g., about 1-5%) for flatness, thermally treated (e.g. by natural ageing or artificial ageing) and optionally subjected to final treatment practices (e.g. a forming operation, paint-bake cycle in case of an automotive application).
The solution heat treatment can be carried out as a batch process or a continuous process. In order to produce 7000-series aluminium alloy sheet material on an industrial scale in an economical attractive manner it is required that sufficiently high line speeds can be maintained while the sheet material is moving through the continuous heat-treatment furnace. However, too high line speeds may impact on the soaking time of the strip material at the required solution heat-treatment temperature and thereby affecting amongst others the mechanical properties of the aluminium strip. Whereas too low line speeds may result in deteriorate features such as HTOD (High Temperature Oxidation Deterioration).
There is consequently a need for an improved process of producing 7000-series alloy sheet products that exhibit fine equiaxed grains and are substantially free from second phase particles.